This is a proposal to study the biogenesis of messenger RNA and its post-transcriptional modification by poly A polymerases. The experimental approach will examine heterogeneous nuclear RNA (hnRNA), its synthesis and processing into messenger RNA in a cultured myeloma cell line which produces a specific messenger RNA for the light chain polypeptide of immunoglobins. Preparations of nuclei from myeloma cells which synthesize RNA in vitro will be used to study the polyadenylation of hnRNA and its processing into mRNA and the release of mRNA from the nucleus. A second related area of investigation involves the detection, location and distribution of small transcribed homopolymer sequences in the hnRNA fraction of HeLa cells. Both oligo A and oligo U sequences of about 25 nucleotide lengths are present in all size classes of hnRNA. An interesting relationship between oligo A and the 3' terminal poly A sequences of hnRNA has been revealed by the observation that the two sequences are not present in the same molecule since methods have been developed for the separation of the two classes of hnRNA. A model for processing of hnRNA that uses these oligo A sequences as priming sites for the addition of the 3' terminal poly A sequence has been proposed and experiments to test it are in progress. Methods are now being sought for the isolation of these oligo A containing RNA molecules in the hnRNA of the nucleus and for the isolation of oligo U containing RNA molecules found in the messenger RNA of HeLa cells.